NOTE ON EMBRITTLEMENT FROM ALKALINE
TIN PLATING

C. A. ZAPFFE AND M. E. HASLEMBaltimore, Md.

In the June, 1950 issue of PLATING,
a research was described in which hydrogen absorption by a steel cathode during
electroplating was measured in terms of the embrittlement manifesting itself
in a special bend test(1). Figs. 3 and 4 of that paper carried data for acid
and alkaline ti baths, respectively. The data for the alkaline bath have
since been called in question by Hedges and Lewis of the Head Office and Laboratories
of the Tin Research Institute in Middlesex, England(2), and by Nekervis at that
Institutes Laboratories in Columbus, Ohio(3). The point of the objection
is that the procedure used in the research did not take advantage of recent
recommendations of the Institute, particularly with regard to temperature and
current density.

TABLE I.
DESCRIPTION OF SPECIMENS

Designation

Analysis, %

Condition*

Stainless Steel
AISI 440-C

C = 1.01
Cr = 16.8

Annealed and cold-drawn
to 24.5 percent reduction of area (133,000 psi tensile strength)

Because the embrittlement from the alkaline bath was of serious proportions,
any advantages offered by altered plating conditions warrant attention. The
research was, therefore, extended to include the recommendations of the Tin
Research Institute. The results comprise the present communication.

REMARKS ON TESTING
Details of the testing procedure can be foud in previous publications of the
authors. The bend machine produced a constant rate of bend around a fixed pin;
the specimen was 16-gauge wire; angles of fracture between 0° and 180°
of bend were measured, a 180° bend representing a lack of embrittlement
so far as the sensitivity of the test is concerned.

Table I contains a description of
the specimens, which included both a stainless steel and a carbon steel of representative
compositions. The stainless steel was from the same lot as used in the preceding
research; but the SAE 1060 steel replaced the carbon steel employed in the previous
research, specimens of which were no longer available.

Table II lists two plating baths,
both of the alkaline sodium stannate type. The one identified as ME
was used in the foregoing research, its formulation coming from Oplinger and
Bauchs article in the 1942 edition of Modern Electroplating(4).
The bath identified as TRI is the one recommended by the Tin Research
Institute(6).

In comparing the two baths, one will
note that they have identical tin concentrations and that the TRI bath
is more alkaline and lacks the sodium acetate and hydrogen peroxide present
in the ME bath. The H2O2 is omitted in the present experiments because the earlier
work(1) had shown it to have no measurable effect upon embrittlement.

RESULTS FOR STAINLESS STEEL
In addition to these differences in bath composition, both current density and
temperature in the previous work differed considerably from the TRI recommendation,
principally because of arbitrary selections on the low side of the Oplinger-Bauch
temperature range and the high side of their current-density range.

Accordingly, three principal variables
were to be explored, namely, (1) bath type, (2) temperature, and (3) current
density.
In Table III, the ME and TRI baths are compared with respect to all three variables.
Four plating periods were used throughout, of which the first represented a
very brief treatment for disclosing immediate effects of the plating, and the
three longer periods serv-ed to outline the general limits which the embrittlement
might reach.

In the first column of Table III,
the current density (37 asf; 4 amp/dm2) is the same as that used in the previous
research, but the temperature is 80°C (176° F) instead of 60°C (140°F).
The data are virtually identical with those shown on the graph of Fig. 4 in
the prior study.

In the second column, the data refer
to the same ME bath, but at the temperature and current density recommended
for the TRI bath. The temperature of 80°C (176°F) is the upper limit
of the range recommended in Modern Electroplating; the current density is toward
the lower end of the recommended range. Bend data for the longer periods
of plating show no significant difference from those of the other tests, although
the 30-second treatment indicates less embrittlement in early stages. This is
consistent with the know loss of plating efficiency at higher current densities.
Nevertheless, numerous researches now conducted on hydrogen behavior during
electroplating make it clear that current efficiency is a far less important
factor in determining hydrogen absorption than is commonly supposed.

In the third column of Table III,
the TRI bath is compared directly with the ME bath in the preceding column,
being operated at identical temperatures and current densities. The data correspond
closely, but they somewhat favor the ME bath, particularly in the early plating
period. This possibly reflects the higher alkalinity of the TRI bath, although
the characteristically increased hydrogenizing power of alkaline baths(1) has
not yet been identified with an alkalinity factor. As for a comparison between
the third and first columns, the data for the embrittlement from the TRI
bath at the recommended: 80°C (176°F) and 15 asf (1.6 amp/dm2) are identical
in all but :one reading with those of the ME bath operating at the same temperature
but at the higher current density of 37. asf (4 amp/dm2).

On the last column of Table III,
the TRI bath is tested at a temperature of 60°C (140°F) merely for purposes
of completing the comparison with the ME bath and of determining the effect
of temperature in the TRI bath. There is a slight indication of improvement,
particularly in the early period, which is consistent with known effects of
temperature on hydrogen absorption(6).

TABLE II. BATH COMPOSITIONS
AND OPERATING CONDITIONS

Bath Components
Operating Data

TRI Bath

ME Bath

Na2SnO3 H20,
g/l

90

90

NaOH, g/l

12.5

7.5

NaC2H3O2, g/l

15.0

H2O2 (20-vol, 6%),
g/l

(2.5)*

Temperature, °C

80

60-80

Current density,
asf

10-25

10-40

Anode

cp stick tin

cp stick tin

*Omitted here;
previous work showed no effect on embrittlement. .

TABLE III.
EMBRITTLEMENT OF STAlNLESS STEEL IN TWO SODIUM STANNATE BATHS Bath temperature
and current density as noted

Plating
Time, min

ME Bath

TRI Bath

T = 80°C

T = 80°C

T = 80°C

T = 80°C

CD = 37 asf

CD = 15 asf

CD = 15 asf

CD = 15 asf

Bend Angles
in Degrees

0

180, 180

180, 180

180, 180

180, 180

0.5

70, 70

90, 90

70, 70

90, 75

4

40, 35

40, 42

40, 40

40, 50

16

30, 30

32, 30

30, 30

32, 30

32

30, 27

30, 30

30, 27

30, 32*

*Treeing

RESULTS FOR CARBON STEEL
Because significant differences have been found between the behaviors of stainless
and carbon steels during hydrogenizing(7), a concluding series of experiments
was conducted specifically to make certain that the foregoing results are valid
for carbon steel as well and to disclose the general scope of the hydrogenizing
to be expected from the TRI bath. Table IV lists the bend angles for plating
periods again chosen in logarithmic progression, but with a 1-minute period
substituted for the 30-second period of the previous tables. The specimens were
tempered to a hardness causing breakage of the blank at an angle of bend of
115 ± 10°.

Embrittlement clearly manifests itself
within the first minute of plating. This effect continues on extended plating,
with the bend angles attaining a considerably lower limit than shown for carbon
steel in Fig. 4 of the previous paper(1); this, however, is a more sensitive
steel, having a lower blank bend value. No data for the ME bath are included,
because extensive previous studies make it obvious that the behavior of the
TRI bath is characteristic of alkaline baths in general and that the data of
Table II suffice for establishing the desired comparison.

CONCLUSIONS
On the basis of these data, one can draw the following conclusions:
(1) There is no significant difference between the ME and TRI baths so far as
hydrogen embrittlement of the basis metal is concerned;
(2) There are indications of very slight advantages for (a) the lower bath temperatures,
(b) the lower current densities, (c) the ME bath as compared to the TRI bath
at identical temperatures and current densities, particularly during brief plating
periods;
(3) Whereas the two preceding conclusions are drawn on the basis of experiments
with Type 440-C stainless-steel wire, tests with the TRI bath on SAE 1060 carbon-steel
wire confirm a similarity in behavior, consistent with that observed in other
grades of steel and comparable to that exhibited by other alkaline tin plating
solutions.

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